Hitchhiking in South America varies a lot. In some countries, such as Ecuador, it is really easy to get rides, especially when you look like a foreigner, though there are also some more dangers involved. Uruguay, Argentina and Chile are considered the safest countries to hitchhike. In Brazil it's a bit less secure, but the buses are much more expensive than in other countries. Hitchhiking in Colombia is considered very dangerous, and on top of that, if you do try, you will find it extremely hard to actually get a ride.

map24 is in Portuguese but has information about the whole of South America.